This invention relates to a method and apparatus for reducing adjacent channel power in wireless communication systems. More particularly, the present invention is directed to a system for digitally and adaptively pre-distorting a signal that includes applying a correction to the signal before it is applied to the input of an amplifier such that the correction is equal and opposite to anticipated distortion produced by the amplifier. Thus, the correction and the amplifier distortion cancel one another resulting in a system with an overall linear transfer characteristic. In these circumstances, the adjacent channel power is desirably reduced.
While the invention is particularly directed to the art of reducing adjacent channel power in wireless communication systems and finds particular application in base stations, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be applied to wireless telephone units and other such devices.
By way of background, modern wireless communication systems require tight control of adjacent channel power (ACP) in order to avoid interference with neighboring channels. This requirement places a heavy burden on transmission power amplifiers of base stations which require a high degree of linearity to avoid generating spurious out-of-channel emissions while maintaining high power efficiency to minimize temperature rise and maximize reliability. These constraints are particularly severe for code division multiple access (CDMA) systems where the pseudo-random nature of the signal results in a peak power which exceeds the average power by a factor of ten or more.
In order to achieve reasonable power efficiency, base station amplifiers are typically designed to operate in class AB mode. Unfortunately, the transfer characteristic of the amplifier departs substantially from linear when in class AB mode. This results in unacceptably high ACP. The traditional method of linearizing the amplifier is to employ adaptive feed-forward which, while effective, results in a substantial cost increase.
The present invention contemplates a new method and apparatus for reducing adjacent channel power in wireless communication systems that resolve the above-referenced difficulties and others.